1. Field of the Invention
The present disclosure relates to an Active Matrix Organic Light Emitting Diode (AMOLED) display device, and more particularly, to a voltage compensation type pixel circuit of an AMOLED display device, which can compensate for a positive threshold voltage and a negative threshold voltage and enables a driving transistor to always operate in a saturation region.
2. Discussion of the Related Art
An AMOLED display device is a self-emitting device to emit light through an organic light emitting layer by electron-hole recombination. The AMOLED display device has high luminance and a low driving voltage and can have an ultra-slim size, thereby being expected as a next-generation display device.
Each of a plurality of circuits constituting an AMOLED display device includes a light emitting element comprised of an organic light emitting layer between an anode and a cathode, and a pixel circuit for independently driving the light emitting element. The pixel circuit may be classified into a voltage-type pixel circuit and a current-type pixel circuit. Since the voltage-type pixel circuit has a simpler external driving circuit than the current-type pixel circuit and is suitable for a high-speed operation, it is well suited to applications to a pixel circuit for an AMOLED TV etc.
The voltage-type pixel circuit mainly includes a switching Thin Film Transistor (TFT), a capacitor, and a driving TFT. The switching TFT charges a voltage corresponding to a data signal to the capacitor in response to a scan pulse, and the driving TFT controls the amount of current flowing into a light emitting element according to the magnitude of the voltage charged to the capacitor, thereby adjusting luminance of the light emitting element. Generally, luminous intensity of the light emitting element is proportional to the current supplied from the driving TFT.
However, a conventional voltage-type pixel circuit has non-uniform luminance due to non-constant threshold voltages Vth of driving TFTs according to position because of deviation in a manufacturing process etc. or has a short lifetime due to a reduction of luminance by varied threshold voltages over time. To solve such a problem, the voltage-type pixel circuit uses a method for detecting and compensating for the threshold voltage of the driving TFT.
A conventional voltage compensation type pixel circuit, which is disclosed, for example, in U.S. Pat. No. 7,649,202 (Korean Patent No. 10-0636483), detects, as a threshold voltage of a driving TFT, a source-gate voltage at which a drain-source current becomes sufficiently small by connecting the gate and the drain, and compensates a data voltage by the detected threshold voltage. The conventional voltage compensation type pixel circuit uses a control TFT serially connected between the driving TFT and a light emitting element in order to cut off light emission of the light emitting element upon detecting the threshold voltage. However, the conventional voltage compensation type pixel circuit is problematic as follows.
First, when a pixel circuit using n-type TFTs detects a threshold voltage of a driving TFT of a diode structure, it cannot detect a negative threshold voltage of the driving TFT. Further, a pixel circuit using p-type TFTs cannot detect a positive threshold voltage of the driving TFT. This is because, in the driving TFT of a diode structure in which the gate and the drain thereof are connected to each other, a gate-drain voltage is 0V and thus a minimum or maximum detectable threshold voltage is limited to 0V.
Second, since the light emitting control TFT serially connected between the driving TFT and the light emitting element always operates in a linear region during light emission, it is greatly affected by bias stress and is greatly subjected to degradation. Generally, if a value obtained by subtracting a threshold value Vth from a gate-source voltage Vgs of a TFT is equal to or less than a drain-source voltage Vds of the TFT (i.e. Vgs−Vth≦Vds), then the TFT is in a saturation region, and if a value obtained by subtracting the threshold value Vth from the gate-source voltage Vgs of the TFT is greater than or equal to the drain-source voltage Vds of the TFT (i.e. Vgs−Vth≧Vds), then the TFT is in a linear region. It is known that TFT degradation progresses rapidly in the linear region. However, in the conventional voltage compensation type pixel circuit, the light emitting control TFT operates in the linear region and the driving TFT operates in the saturation region, during a light emitting period. Accordingly, the light emitting control TFT is subjected to degradation faster than the driving TFT due to bias stress.
Meanwhile, if the light emitting control TFT is omitted in order to solve such a problem thereof, since the light emitting element emits light even during a non-light emitting period, black luminance is increased and thus contrast is lowered.
As a known prior art document related to the invention of the present application, for example, we note Korean Patent No. 10-0636483 (U.S. Pat. No. 7,649,202).